1. Field of the Invention
The present invention relates to differential line drivers, and more particularly, to low distortion high speed differential drivers.
2. Related Art
Transmit line driver amplifiers typically need to drive a very low ohmic loads and need to be stable and operate with a high linearity in a variety of feedback configurations. These can include pure resistive gain configurations, or configurations with gain and some filtering embedded in the feedback network (see FIG. 1). As shown in FIG. 1, a line driver amplifier configuration includes an amplifier 101 having differential outputs VOP, VON driving a load 103. Differential inputs to the amplifier 101 include VIP and VIN, optionally passing through an impedance feedback network 102A, 102B.  Note that in FIG. 1, the power supply and ground connections are not shown, since they are outside of the signal path.
To maintain high linearity, the open-loop amplifier bandwidth needs to be kept high, so that the amplifier 101 has a high gain at a given frequency to suppress the open-loop non-linearities in the amplifier 101. In such high bandwidth designs, feedback is important to the stable operation of the line driver. Since the line driver has to support a variety of feedback networks, the amplifier 101 alone is designed and fabricated separately on a chip (without any feedback network). The amplifier 101 inputs VIN, VIP, outputs VON, VOP and power connections (not shown in FIG. 1) are brought outside the package, to be used on a circuit board in any number of the possible feedback configurations.
The package adds parasitic series resistance Rp, series inductance Lp, and shunt capacitance Cp (see FIG. 2). FIG. 2 illustrates the conventional packaged amplifier 101. As shown in FIG. 2, the amplifier 101 is packaged within a package 201, which includes various parasitics RP, LP and CP, shown in FIG. 2 in the form of a lumped model. Note that other models may be used, and actual behavior of the package parasitics typically is more complex than is illustrated in FIG. 2. However, the lumped model of FIG. 2 generally gives a good first-order approximation of package parasitic-behavior.
An example of the impedance network 102 could be a summing junction using resistors. The impedance network 102, in actual use, can vary depending on the package 201, the application, and a number of other factors. Typically, the same amplifier 101 needs to work in multiple configurations, with good stability. Also, it is generally not desirable to restrict a particular application to using a specific package. Alternatively, it may be desirable to use a cheaper package with greater variation of a package parasitics, or with higher package parasitics.
Because of the package parasitics, and depending on the load 103 and the nature of the impedance feedback network 102A, 102B, additional phase loss may be introduced into the feedback path of the amplifier 101. Thus, stability becomes a problem for such high bandwidth amplifiers due to the introduction of additional phase (especially for low closed loop operation). The extra phase loss due to the parasitics limits the bandwidth that can be achieved by the amplifier 101. What this means in practice is that to achieve a certain amount of linearity at a certain frequency, one needs a certain amount of gain to overcome the open loop non-linearities of amplifier 101. As the frequency increases, the gain decreases by 20 dB per decade for a single-pole response. Thus, it is desirable to reach a higher bandwidth (i.e., a higher input signal frequency). The package parasitics also become a bigger problem at higher frequencies.
The VOP, VON outputs of the amplifier 101 have to go through two sets of parasitic elements, apart from the external feedback network 102A, 102B before reaching the VIN, VIP terminals of the packaged amplifier 201. This causes significant phase shift at high frequencies and results in instability, which can manifest itself as oscillation, or ringing, resulting in poor or unacceptable performance.
A conventional approach provides an external pin for compensation control for the amplifier 101. Some general-purpose amplifiers have this feature. However, this requires an extra terminal to be brought out to the package 201, which is not desirable, and which can also add board noise back into the chip.
Accordingly, there is a need in the art for a line driver/amplifier that operates at high frequencies with a high degree of linearity.